<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    
</body>
<script>
    var n = parseInt(prompt("请输入一个值，计算1+2+3+···+n"))
    //简便方法   n(n+1)/2
    // alert("1+2+3+···+ " + n +"="+ (n * (n+1) / 2))
    var sum = 0;
    for(var i = 1;i <= n ; i++){
        sum += i;
    }
    alert(sum)
</script>
</html>